5 Answers
5

Web

Some websites are better than others for this, but there are some great resources out there for clone recipes. There are the "industry" sites, like BYO, that list recipes. There are forums, like Homebrew Talk, that have great discussions about various clones. And there are recipe sharing sites, like BeerTools.com and Beer Calculus, that have some great variations and consistent details.

Magazines

Some magazines, like Brew Your Own and Zymurgy, provide clone recipes, as well as other recipes for the readers to try. This is a great resource, as they usually delve deeper than just the ingredients. They tend to explain the why of each choice, process, etc.

Trial and Error

Understanding what goes into beer at a detailed level allows a brewer to taste a beer, look at the color, etc., and make educated guesses on how to mimic it. This is definitely advanced, but with practice, this is a great way to clone a beer.

Books

Podcasts

Podcasts like those put on by The Brewing Network, and in particular The Jamil Show are a good resource for clone recipes. The show's format changed a couple years back to what is now called "Can You Brew It," in which a listener will request the hosts clone a commercial brew. They'll speak with the commercial brewer, get tips and often the actual recipe, then they'll try their best to clone it. It is very informative and entertaining.